Method for the production of n,n-dimethylacetamide (dmac)

ABSTRACT

A process for preparing N,N-dimethylacetamide (DMAC) by continuously reacting methyl acetate (MeOAc) with dimethylamine (DMA) in the presence of a basic catalyst, wherein MeOAc is used in the form of a methanolic solution which is obtained as a byproduct in the preparation of polyTHF by transesterifying polyTHF diacetate with methanol.

The present invention relates to a process for preparingN,N-dimethylacetamide (DMAC) by continuously reacting methyl acetate(MeOAc) with dimethylamine (DMA) in the presence of a basic catalyst.

DMAC finds use as a polar solvent, for example for polymers and forgases, as paint removers, extractants, catalysts and crystallizationassistants. In the coatings industry, DMAC is used, owing to its highboiling point, for specific coating materials based on polymericbinders, for example polyamides and polyurethanes, DMAC is also used forproducing fibers and films and as a reaction medium. In the spinning ofSpandex® fibers, DMAC is used as an assistant and subsequently recoveredat least partly.

DMAC may be prepared from acetic acid and dimethylamine, for exampleaccording to FR-A-1,406,279.

Carboxamides are also obtainable by aminolysis of correspondingcarboxylic esters; cf., for example, ‘Organikum’, VEB Deutscher Verlagder Wissenschaften, 1963, pages 374-375.

The article by J. P. Guthrie in J. Am. Chem. Soc. 96, pages 3608-15(1974) relates to reaction kinetics and thermodynamics aspects ofreactions including the aminolysis of carboxylic esters.

CA-A-1 073 467 and CA-A-1 073 468 (both General Electric Comp.) describethe preparation of diols and N,N-dialkylamides by reacting carboxylicdiol esters with dialkylamines.

U.S. Pat. No. 4,258,200 (Air Products) teaches the preparation of DMACfrom methyl acetate and DMA in the presence of cobalt catalysts.

In Example 1, a “20% methanol-methyl acetate azeotrope” is used for thereaction at 155-160° F. (68.4-71.2° C.).

JP-A-02 160749 (Lion Akzo KK) relates, according to the Patent Abstractsof Japan, to the reaction of aliphatic carboxylic esters with ammonia oran amine, such as monomethylamine, ethylenediamine, diethylenetriamine,in the presence of an “alkali catalyst” at from 50 to 180° C., inparticular from 80 to 160° C., and in the pressure range from standardpressure to 9.81 bar (10 kg·cm⁻²·G).

From 0.1 to 10 mol %, in particular from 1 to 5 mot %, based on thecarboxylic ester used, of sodium methoxide (NaOMe) is used as thecatalyst.

Derwent Abstract 84-016399/03 (SU-A-1 004 357; Dnepr Chem. Techn. Inst.)describes the preparation of DMAC or dimethylformamide (DMF) by reactinga 5-20% excess of corresponding methyl carboxytate in methanol with DMAat 50-150° C. and subsequently recycling unreacted ester and methanolinto the reaction stage.

In the example, a solution of 0.4 kg of methyl formate in 0.2 kg ofmethanol/h is reacted continuously with 0.2 kg of vaporous DMA/h to giveDMF.

The two German patent applications No. 102004030616.8 of Jul. 24, 2004and DE-A-10 315 214 to BASF AG relate to processes for purifying DMAC.

It is an object of the present invention to provide an improved,economically viable, selective, energy-saving and non-resource-intensiveprocess for preparing N,N-dimethylacetamide (DMAC). The process shouldafford DMAC in high yield and space-time yield and in high purity (forexample free or virtually free of acetic acid, high color quality).

Accordingly, a process has been found for preparingN,N-dimethylacetamide (DMAC) by continuously reacting methyl acetate(MeOAc) with dimethylamine (DMA) in the presence of a basic catalyst,which comprises using MeOAc in the form of a methanolic solution whichis obtained as a by-product in the preparation of polyTHF bytransesterifying polyTHF diacetate with methanol.

The process according to the invention can be performed as follows:

For DMAC synthesis, dimethylamine (DMA) is reacted continuously with amethanolic solution of methyl acetate (MeOAc), a secondary stream ofpolyTHF preparation.

Preference is given to using in the range from 0.2 to 2.0 mol,particularly from 0.5 to 1.5 mol, very particularly from 0.8 to 1.2 mol,for example from 0.9 to 1.1 mol or from 1.0 to 1.05 mol, ofdimethylamine (DMA) per mole of methyl acetate.

The DMA used preferably has a purity of ≧99% by weight, in particular≧99.4% by weight, and is, for example, in the range from 99.5 to 99.8%by weight.

The methanolic MeOAc solution preferably has a concentration in therange from 65 to 90% by weight, preferably from 70 to 85% by weight, inparticular from 75 to 82% by weight, of MeOAc.

In a particular embodiment of the invention, the methanolic MeOAcsolution used is a corresponding by-product stream which is obtained inthe production of polyTHF (polytetrahydrofuran), for example by thetwo-stage BASF process according to EP-A-3112, DE-A-197 58 296 and/orDE-A-198 17 113.

The methanolic MeOAc solution is obtained as a corresponding by-productstream in the distillative workup, for example, in the form of a methylacetate/methanol azeotrope (boiling point: 54° C./1013 mbar), sincestoichiometric amounts of MeOAc are formed in the transesterification ofpolyTHF diacetate (=poly-(1,4-butanediol) bis(acetate)) with methanol togive polyTHF.

The methanolic MeOAc solution preferably has the following contents:

-   MeOAc: from 65 to 90% by weight, preferably from 70 to 85% by    weight, in particular from 75 to 82% by weight,-   Methanol: from 10 to 30% by weight, preferably from 14.8 to 25% by    weight, in particular from 17.6 to 22% by weight,-   Dimethyl ether: from 0 to 2% by weight, preferably from 0.1 to 1.5%    by weight, in particular from 0.2 to 1.2% by weight,-   THF: from 0 to 4% by weight, preferably from 0.1 to 3.5% by weight,    in particular from 0.2 to 1.5% by weight, and-   H₂O: from 0 to 0.1% by weight, preferably from 0 to 0.01% by weight,    in particular from 0 to 0.003% by weight.

In particular, the methanolic MeOAc solution consists of MeOAc, MeOH,dimethyl ether, THF and water in the above-specified amounts.

The continuous reaction is preferably carried out at an absolutepressure in the range from 1 to 200 bar, preferably from 3 to 100 bar,in particular from 10 to 30 bar, very particularly from 12 to 25 bar,for example from 15 to 20 bar.

The reaction temperature is preferably in the range from 20 to 200° C.,preferably from 60 to 140° C., in particular from 80 to 120° C., veryparticularly from 90 to 110° C., for example from 95 to 105° C.

Useful reactors for the inventive reaction are in particular backmixedreactors, for example stirred tank reactors or jet loop reactors,nonbackmixed reactors such as stirred tank batteries or tubularreactors, and special designs such as reaction columns with and withoutinternal or external delay volumes, in which internal and external heatremoval is possible.

The reaction is effected with particular preference in a jet loopreactor. The jet loop reactor is preferably equipped with an insert tubeand nozzle at the bottom. Preference is given to adding DMA togetherwith the catalyst through the circulation-pumped driving jet and theMeOAc through the outer jet.

To complete the conversion, particular preference is given to attachingdownstream of the main reactor, for example the jet loop reactor, apostreactor, for example a flow tube or a cascaded delay vessel.

The reactor types mentioned are known to those skilled in the art, forexample, from Ullmanns Enzyklopädie der Technischen Chemie, 4th edition,volume 13, p. 135 ff., and P. N. Rylander, “Hydrogenation andDehydrogenation” in Ullmann's Encyclopedia of Industrial Chemistry, 5thed. on CD-ROM.

In the process according to the invention, the basic catalyst used ispreferably an alkali metal hydroxide, alkaline earth metal hydroxide,alkali metal alkoxide, alkaline earth metal alkoxide, alkali metalcarbonate, alkaline earth metal carbonate, alkali metalhydrogencarbonate, alkaline earth metal hydrogencarbonate and/or anamine, in particular tertiary amine.

The alkali metal is Li, Na, K, Rb or Cs, in particular Na or K.

The alkaline earth metal is Be, Mg, Ca, Sr or Ba, in particular Mg orCa.

The alkoxide is preferably a C₁₋₄-alkoxide, in particular methoxide.

The amine, especially an aliphatic amine, is preferably aC₃₋₁₂-alkylamine, for example triethylamine, tri-n-propylamine,tri-n-butylamine, dimethylethylamine, diethylmethylamine,N-methylpiperidine, triethylenediamine (TEDA).

In the process according to the invention, no cobalt catalysts accordingto U.S. Pat. No. 4,258,200 are used.

A very particularly preferred catalyst in the process according to theinvention is sodium methoxide (NaOMe).

The catalyst is present in the reaction mixture in homogeneous and/orsuspended form.

In the continuous process, preference is given to using in the rangefrom 0.0002 to 0.09 mol, preferably from 0.002 to 0.05 mot, inparticular from 0.003 to 0.02 mol, of the catalyst or catalyst mixtureper mole of methyl acetate used.

The catalyst or the catalyst mixture is advantageously used in the formof a solution and/or suspension in a solvent or suspension medium.

Preferred solvents and/or suspension media are water and alcohols (e.g.C₁₋₄-alcohols such as methanol, ethanol, n-propanol, n-butanol) ormixtures thereof.

In the case of an alkali metal alkoxide as the catalyst, preference isgiven to dissolving the alkali metal alkoxide in the alcohol whichcorresponds to the alkoxide by protonation.

The catalyst or the catalyst mixture is used in the abovementionedpreferred amounts, preferably in the form of from 1 to 35% by weight, inparticular from 5 to 30% by weight, solution or suspension.

Particularly advantageously, the catalyst used is NaOMe in theabove-mentioned preferred amounts in the form of a methanolic solution,in particular in the form of a from 1 to 35% by weight solution, veryparticularly in the form of a from 25 to 30% by weight solution.

The reaction of the MeOAc in the process according to the invention ispreferably carried out in the presence of less than 1% by weight,particularly less than 0.5% by weight, very particularly in the rangefrom 0 to 0.3% by weight, of water, based in each case on the weight ofthe two feedstocks, MeOAc and DMA (in total).

The heat of reaction is removed preferably via an external heatexchanger. Particularly advantageously, the steam raised in the externalheat exchanger, for example 1.5 bar steam, is utilized in a synthesisplant for methylamines from methanol and ammonia.

In the process according to the invention, the liquid reactor effluentfrom the synthesis stage consists of

in the range from 45 to 74.5% by weight, particularly from 50 to 70% byweight, of DMAC,in the range from 25 to 45% by weight, particularly from 29 to 40% byweight, of methanol anda total of from 0.5 to 6% by weight, particularly from 1 to 5% byweight, of DMA, methyl acetate, catalyst (for example sodium methoxide),if appropriate catalyst solvent/suspension medium and by-products.

As the result of the use of methanolic MeOAc solution which is obtainedin the production of polyTHF, tetrahydrofuran (THF) and/or dimethylether may be such by-products.

For further workup, the liquid reactor effluent may be decompresseddirectly into a boiler of a distillation column.

In a particular embodiment, decompression is effected into twoalternately operated distillation boilers.

Advantageously, water or an aqueous or anhydrous protic acid such assulfuric acid, methanesulfonic acid, carboxylic acid (e.g.C₁₋₄-carboxylic acid), in particular phosphoric acid, is added to theeffluent, preferably in an amount which ensures full conversion of thebasic catalyst used to the corresponding acid and to the correspondingalkali metal, alkaline earth metal or ammonium salt of the protic acid.In other words, preference is given to fully neutralizing the basiccatalyst used and present in the reactor effluent by reacting with H⁺.

This is advantageous since the basic catalyst, for example sodiummethoxide, would catalyze the dissociation of DMAC after the outgassingof residual DMA.

The organic product mixture is preferably removed from salts present byevaporation (at standard pressure or under reduced pressure, for examplein a reboiler), for example until a salt which precipitates outdistinctly reduces the heat exchanger output and leads to encrustations.

The boiler for the reactor effluent is then preferably changed and theresidue of the old boiler is concentrated as far as possible byevaporation. The precipitated solid salt residue may be dissolved inwater and disposed of as a solution in a water treatment plant.

The reactor effluent which has been evaporated off from the solid andpartially or totally condensed is worked up by distillation, for examplein one, two, three, four or more columns which are connected to oneanother if appropriate.

Preference is given to effecting the workup in three continuousdistillation columns.

In one column A, methanol and any other low boilers (DMA, water, THF,methyl acetate, inter alia) are removed overhead at preferably from 0.8to 1.2 bar.

In the distillation column D, preferably connected downstream, for lowboiler purification, an aqueous or anhydrous methanol stream which maycomprise DMA is enriched and is, for example, advantageously recycledfor use in a methylamine synthesis plant (in particular for DMApreparation).

The bottom effluent of column A is fed to a column B. At preferably100-500 mbar abs., pure DMAC (≧99.5% by weight, in particular ≧99.7% byweight, very particularly ≧99.8% by weight, for example in the rangefrom ≧99.9 to 99.99% by weight) is removed here, preferably via a liquidside draw which is disposed preferably in the rectifying section.

The top effluent of column B, comprising DMAC (e.g. 98% by weight ofDMAC, in particular from 98.5 to 99.5% by weight of DMAC), is preferablyrecycled into column A.

The bottom effluent of column B is separated once more in a column C,preferably at standard pressure, and the top effluent comprising DMACand methanol (e.g. approx. 94% by weight of DMAC and approx. 6% byweight of methanol) is preferably likewise recycled to column A and thebottom effluent of column C (high boilers, DMAC and added methanol)passes to disposal, for example incineration. The third column Cdistinctly reduces the amount of residue.

The distillative purification of DMAC may also be effected according toone of the processes of the two German patent applications102004030616.8 of Jul. 24, 2004 and DE-A-10 315 214 (both BASF AG).

It has been recognized in accordance with the invention that the processmay advantageously also be carried out in a plant which has originallybeen designed for the preparation of N,N-dimethylformamide (DMF) fromcarbon monoxide (CO) and DMA.

Slight modifications/plant improvements (for example postreactor, tankfor DMAC and/or relating to the column connection) thus advantageouslyallows both DMF and DMAC, for example in alternating operation, to beprepared in the DMF plant as described, for example, in K. Weissermel,H.-J. Arpe, Industrielle Organische Chemie, Wiley-VCH, 5th edition 1998,page 49, or in general and in principle in JP-A2-110 92 434. In otherwords, the invention also enables the alternative or alternatingproduction of DMAC in a DMF plant.

It is possible by the process according to the invention to achieve DMACyields in the range of ≧88%, in particular ≧95%, very particularly ≧99%,for example from 99.5 to 99.9% (based in each case on MeOAc used), atMeOAc conversions in the range of ≧90%, in particular very particularly≧99%, for example from 99.5 to 100%.

The DMAC space-time yields are in the range from 0.1 to 0.85 kg ofDMAC/(liter of reactor volume·h), for example from 0.2 to 0.5 kg ofDMAC/(liter of reactor volume·h).

The process according to the invention affords DMAC with a purity of≧99.5% by weight, in particular ≧99.7% by weight, very particularly≧99.8% by weight, for example in the range from to ≧99.99% by weight(see below for method and conditions for purity determination),

a water content ≦200 ppm, for example in the range from 50 to 150 ppm(to DIN 51777), anda Pt/Co color number ≦10, particularly ≦8, for example in the range from1 to 6 (to DIN ISO 6271).

The acid content (calculated as acetic acid) of the DMAC is inparticular ≦80 ppm, very particularly ≦70 ppm, for example in the rangefrom 5 to 60 ppm (to DIN 53402).

All ppm data in this document relate to the weight (ppm by weight).

EXAMPLES Example 1

For the one-stage DMAC, synthesis, 45.0 g/h of dimethylamine (DMA) werereacted at 20 bar and 120° C. with 95.5 g/h of methanolic methyl acetate(77.5% by weight) which had been obtained beforehand as a by-productstream in the production of polyTHF according to EP-A-3112, DE-A-197 58296 and/or DE-A-198 17 113 (THF content 1.5% by weight). The watercontent in the feed (DMA+methanolic methyl acetate) was 109 ppm.

The reaction was effected in a loop reactor with a mean residence time(MRT) of 1 h and sodium methoxide (0.48 g/h) in methanolic solution (30%by weight) as the homogeneous catalyst. The heat was removed via anexternal heat exchanger. The energy removed in the external heatexchanger can raise 1.5 bar steam.

The liquid effluent from the synthesis stage consisted of 57.7% byweight of DMAC, 34.2% by weight of methanol, 5.0% by weight of methylacetate and a total of 3.1% by weight of DMA, tetrahydrofuran, sodiummethoxide and by-products.

Example 2

All settings from Example 1 were adopted. However, the water content ofthe feed stream was 550 ppm. After a short time, there were blockages inthe reactor as a result of precipitated sodium acetate, and theexperiment had to be stopped.

Example 3

For the two-stage DMAC synthesis, 45.2 g/h of dimethylamine (DMA) werereacted with 92.5 g/h of methanolic methyl acetate (78.8% by weight)which had been obtained beforehand as a by-product stream in theproduction of polyTHF according to EP-A-3112, DE-A-197 58 296 and/orDE-A-198 17 113 (THF content: 1.0% by weight), at 20 bar and 120° C.

The reaction was effected in a loop reactor with a mean MRT of 1 h andsodium methoxide (0.56 g/h) in methanolic solution (30% by weight) asthe homogeneous catalyst. The heat was removed via an external heatexchanger. The energy removed in the external heat exchanger cangenerate 1.5 bar steam.

The liquid effluent from the synthesis stage consisted of 53.9% byweight of DMAC, 36.3% by weight of methanol, 3.9% by weight of methylacetate and a total of 5.9% by weight of DMA, tetrahydrofuran, sodiummethoxide and by-products.

This effluent was conveyed in straight pass through a tubular reactor at120° C., 20 bar and a mean MRT of 1 h. The effluent consisted of 58.3%by weight of DMAC, 37.3% by weight of methanol, 1.1% by weight of methylacetate and a total of 3.3% by weight of DMA, tetrahydrofuran, sodiummethoxide and by-products.

Example 4

10% by weight of H₂O, a superstoichiometric amount relative to thecatalyst, was added continuously to a reaction effluent according toExample 3 in order to replace the sodium methoxide. In a continuousevaporation still, all volatile constituents (1.8 kg/h) were distilledoff at 135° C. The salt residue (245 g) which had been collected within20 operating hours and had been concentrated to dryness in the bottom ofthe still was dissolved in 1.5 kg of H₂O and removed without residuefrom the still into wastewater in need of treatment.

Example 5

A reaction effluent according to Example 3 was admixed continuously with85% phosphoric acid for the stoichiometric formation of Na₂HPO₄. Oncompletion of catalyst decomposition and evaporation of the volatileconstituents according to Example 4, 400 g/h of the condensed mixturewere fed continuously to a distillation column, and a high boiler stream(218 g/h) comprising 99.2% by weight of DMAC and 0.8% by weight ofby-products was drawn off at a bottom temperature of 175° C. In asubsequent continuous distillation, this stream was worked up further,and 198 g/h of DMAC with a purity of 99.9% were obtained from a sidedraw.

1-21. (canceled)
 22. A process for preparing N,N-dimethylacetamide(DMAC) comprising the step of continuously reacting methyl acetate(MeOAc) with dimethylamine (DMA) in the presence of a basic catalyst,wherein said MeOAc is in the form of a methanolic solution obtained as aby-product in the preparation of polyTHF via transesterification ofpolyTHF diacetate with methanol.
 23. The process according to claim 22,wherein said reaction is carried out at a temperature in the range from80 to 140° C.
 24. The process according to claim 22, wherein saidreaction is carried out at an absolute pressure in the range from 3 to30 bar.
 25. The process according to claim 22, wherein said methanolicMeOAc solution comprises from 70 to 85% by weight of MeOAc, from 14.8 to25% by weight of methanol, from 0.1 to 1.5% by weight of dimethyl ether,from 0.1 to 3.5% by weight of tetrahydrofuran (THF), and from 0 to 0.01%by weight of water.
 26. The process according to claim 22, wherein saidmethanolic MeOAc solution comprises from 75 to 82% by weight of MeOAc,from 17.6 to 22% by weight of methanol, from 0.2 to 1.2% by weight ofdimethyl ether, from 0.2 to 1.5% by weight of THF, and from 0 to 0.003%by weight of water.
 27. The process according to claim 22, wherein saidcatalyst is sodium methoxide.
 28. The process according to claim 27,wherein said catalyst is in the form of a methanolic solution.
 29. Theprocess according to claim 22, wherein said catalyst is present in therange of from 0.0002 to 0.09 mole per mole of MeOAc.
 30. The processaccording to claim 22, wherein said reaction is carried out in a jetloop reactor.
 32. The process according to claim 30, wherein said jetloop reactor comprises an insert tube and a nozzle located at the bottomof said jet loop reactor.
 32. The process according to claim 22,comprising the additional step of neutralizing said catalyst present inthe reactor effluent after the reaction and before distillative workupby reacting said catalyst with a protic acid or by decomposing saidcatalyst with water.
 33. The process according to claim 32, wherein saidprotic acid is phosphoric acid.
 34. The process according to claim 32,comprising the additional step of removing the organic product mixtureafter neutralization with a protic acid and before distillative workupby evaporating said organic product mixture from salts present after thereaction.
 35. The process according to claim 22, comprising theadditional step of continuously distillatively working-up reactioneffluent in a column A, wherein methanol and any other low boilers areinitially removed overhead in column A, followed by feeding the bottomeffluent of column A to a column B, wherein DMAC is removed via a sidedraw, and wherein said DMAC has a purity of greater than or equal to99.7% by weight.
 36. The process according to claim 35, wherein saidDMAC is removed in column B via a liquid side draw which is disposed inthe rectifying section of column B.
 37. The process according to claim35, wherein the top effluent of column B comprises DMAC and is recycledback into column A.
 38. The process according claim 35, wherein thebottom effluent of column B is separated into a column C, wherein thetop effluent of column B is recycled into column A, and wherein said topeffluent comprises DMAC and methanol.
 39. The process according to claim35, wherein the top effluent of column A comprises methanol and ispurified in a column D.
 40. The process according to claim 22, whereinthe DMAC prepared has a purity of greater than or equal to 99.7% byweight, a water content of less than or equal to 200 ppm, and a Pt/Cocolor number less than or equal to
 10. 41. The process according toclaim 22, wherein the DMAC prepared has an acid content calculated asacetic acid of less than or equal to 80 ppm.
 42. The process accordingto claim 22, wherein said process is carried out in a plant wherein DMFcan also be prepared from carbon monoxide and DMA.